Among the different manufacturing methods used, mention may be made of those using steps for bonding and transferring layers. An example of such a method is described hereafter.
According to this method, an embrittlement area is generated within a first so-called “donor” substrate, covered with an insulating layer, for example, by implantation of atomic and/or ionic species. This substrate is then adhered by molecular adhesion to a second so-called “receiver” substrate.
Next, the donor substrate is separated into two portions, along this embrittlement area, for example, by detachment annealing (“exfoliation” or “splitting”), so that an intended thickness of the material of the donor substrate, as well as the insulator layer, are transferred onto the receiver substrate.
Reference may, for example, be made on this subject to the method known under the registered trade mark “UNIBOND®” of the applicant.
The appended FIG. 1 is a graphic illustration of a photograph taken with a scanning electron microscope, which shows a cross-sectional view of the edge of an SOI substrate immediately after its detachment annealing. This SOI substrate comprises a buried insulator layer 2 between a silicon support 1 and a surface silicon layer 3. It is clearly seen that a side edge 20 of the insulator layer 2 (here silicon oxide) is potentially widely exposed to chemical etching.
Now, additional treatments are frequently carried out on such an SOI (or SeOI) substrate notably for improving its surface condition, in particular, for reducing its roughness or for strengthening the adhesive bond between two layers. Such treatments often involve chemical etching, for example, with hydrofluoric acid (HF).
Such a treatment applied to an SOI substrate, as the one illustrated in FIG. 1, results in delamination of the substrate by side etching of the insulator layer 2.
In order to overcome this problem, a method for treating an SOI substrate with Rapid Thermal Annealing (known as “RTA” to a person skilled in the art) is known from U.S. Pat. No. 6,939,783, this treatment having the effect of generating a side encapsulation of the edge of the silicon oxide layer 2 with the silicon located above.
The appended FIG. 2 is a graphic illustration of a photograph taken with a scanning electron microscope, which shows a transverse sectional view of the edge of an SOI substrate at the end of such a treatment. As this may be seen, the silicon of the surface layer 3 of the SOI is moved and covers the side edge 20 of the silicon oxide 2 by protecting the latter. This encapsulation portion is referenced as 30.
Now, the applicant carried out tests and unfortunately noticed that the expected protective beneficial effects of the encapsulation were not always obtained since the edge of the transferred silicon layer 3 may have irregularities relative to the ideal case illustrated in FIG. 2.
The applicant discovered that defects, present at the edge of an SeOI or SOI substrate and although being part of an exclusion area within which no electronic component will be produced, may perturb the encapsulation phenomenon.
Among the latter, a first type of defect designated as “flakes” is formed by pieces of the substrate that have appeared during detachment and that have been detached and then again notably adhesively bonded onto the edge of the substrate.
Such flakes 31 are visible in the appended FIG. 3, which schematically illustrates an SOI substrate seen as a transverse cross-sectional view.
A second type of defect called “jagged edge” is formed by extensions of the surface of the upper layer of silicon beyond the usual edge of the transferred surface.
Such defects 32 are visible in the aforementioned FIG. 3 and in the appended FIG. 4, which is a graphic illustration of a photograph of an SOI substrate edge as seen from above taken with a scanning electron microscope.
The applicant thus discovered that the obtained encapsulation, when it is achieved by an RTA treatment, may be perturbed, notably may be too thin at the top of the jagged edges 32. When this encapsulation is carried out by epitaxy treatment, it may also be perturbed, or even completely anarchical at the flakes 31 on which a larger amount of encapsulation material may be formed with, for example, the formation of material excrescences.
The encapsulation may not even be complete, depending on the shape of the irregularities present at the edge of the substrate (plate edge) and, consequently, certain portions of the buried insulator layer 2 may remain accessible to chemical etching, for example, with hydrofluoric acid (HF).
Moreover, certain flakes or jagged edge pieces may be detached from the receiver substrate before the encapsulation and be redeposited on the surface layer 3 of the SeOI substrate, and then be definitively sealed by the encapsulation, thereby generating defects the surface layer 3.
The appended FIG. 5 is an illustration of a photograph taken with a scanning electron microscope of a substrate of the SOI type, on which this poor quality encapsulation may be observed and, notably, the portion of the poorly encapsulated buried insulator that has undergone etching by hydrofluoric acid.